Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C1/00—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
  • B24C1/003—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods using material which dissolves or changes phase after the treatment, e.g. ice, CO2
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C1/00—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
  • B24C1/08—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods for polishing surfaces, e.g. smoothing a surface by making use of liquid-borne abrasives

Definitions

• TITLE Method and Equipment for removal of ceramic coatings by solid C02 blasting. Background
• Ceramic thick coatings are defined as the protective layer with a thickness greater than 100 ym while the thin films are defined as protective layers with a thickness lower than 100 ym.
• Ceramic thick coatings are made by thermal spray technologies as Air Plasma Spray (APS) , Vacuum Plasma Spray (VPS), Suspension Plasma Spray (SPS) , Solution Precursor Plasma Spray (SPPS) and High Velocity Oxygen Fuel (HVOF) , mainly.
• Ceramic Thin Films are applied by Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) , mainly.
• Thick Ceramic Coatings are used for different applications
• Coatings to improve component wear resistance as A1 2 0 3 , Cr 2 0 3 , AI2O3-T1O2, Al 2 0 3 -Zr0 2 -Ti0 2 ;
• Coatings to improve the component corrosion resistance as A1 2 0 3 , Al 2 0 3 -Ti0 2 , Cr 2 0 3 , Zr0 2 -CaO;
• Thermal Barrier Coatings are composite coatings systems.
• TBC systems consist of (i) a Bond Coat (BC) of MCrAlY alloy (where "M” can be Ni, Co or a combination of both) and (ii) a ceramic Top Coat (TC) of Yttria Partially Stabilized Zirconia (YPSZ) .
• MCrAlY coatings are able to protect the substrates from high temperature oxidation and hot corrosion.
• Zirconia coating for its low thermal conduction coefficient is able to reduce the service temperature at the substrate surface in combination with a cooling gas system. For these reasons TBC systems are applied on gas turbine hot parts.
• MCrAlY alloys by low pressure plasma spray (LPPS) or vacuum plasma spray (VPS) .
• LPPS low pressure plasma spray
• VPS vacuum plasma spray
• APS air plasma spray
• HVOF high velocity oxygen fuel
• the thermally sprayed ceramic TC adhesion is mainly determined by the BC roughness that must have an Ra greater than 10 ym (about 12-16 ym) to guarantee a good thermal fatigue resistance during the component service life.
• Ceramic TC in YPSZ is applied on metallic BC by APS;
• Ceramic Thin Films are applied for different application :
• Thin films to modify optical properties of the component Thin films to modify optical properties of the component ;
• layer is the first operation step
• the main characteristic of the stripping processes is to remove the coating without damaging the substrate characteristics (avoiding corrosion, geometrical variations, etc.).
• the Thermal Barrier Coating removal is a good example to understand the stripping process.
• the TC and/or BC stripping is necessary on new coated parts during MCrAlY or TBC manufacturing to correct problems of coating quality and during repair operations on serviced coated components.
• the removing of the TBC system is very time consuming and expensive: If it is necessary to remove only the ceramic TC, it is necessary to remove all the TBC using sand blasting to strip the ceramic top coat and chemical acid attack to remove the metallic MCrAlY BC . This procedure is necessary because sand blasting decreases bond coat roughness which is fundamental for TC adhesion. So, in the end, it is necessary to strip both the coatings to fix only the top coat. This leads to have very high re-work and environmental costs.
• Dry-ice particle blasting is similar to sand blasting, plastic bead blasting, or soda blasting where a media is accelerated in a pressurized air stream (or other inert gas) to impact the surface to be cleaned or prepared.
• the media that impacts the surface is solid carbon dioxide (C0 2 ) particles.
• C0 2 solid carbon dioxide
• One unique aspect of using dry-ice particles as a blast media is that the particles sublimate (vaporize) upon impact with the surface. The combined impact energy dissipation and extremely rapid heat transfer between the pellet and the surface cause instantaneous sublimation of the solid CO 2 into a gas.
• the gas expands to nearly eight hundred times the volume of the particle in a few milliseconds in what is effectively a "micro-explosion" at the point of impact that aids the coating removal process. Because of the CO 2 vaporizing, the dry-ice blasting process does not generate any secondary waste. All that remains to be collected is the removed coating.
• the kinetic energy associated with dry-ice blasting is a function of the particle mass density and impact velocity. Since CO 2 particles have a relatively low density, the process relies on high particle velocities to achieve the needed impact energy. The high particle velocities are the result of supersonic propellant or air-stream velocities.
• the CO 2 particles have a very low temperature of -109°F (-78.5°C). This inherent low temperature gives the dry-ice blasting process unique thermodynamically induced surface mechanisms that affect the coating or contaminate in greater or lesser degrees, depending on coating type. Because of the temperature differential between the dry ice particles and the surface being treated, a phenomenon known as thermal shock can occur. As a material's temperature decreases, it becomes brittle, enabling the particle impact to break-up the coating and sever the chemical bond that is weakened by the lower temperature. The thermal gradient or differential between two dissimilar materials with different thermal expansion coefficients can serve to break the bond between the two materials. This thermal shock is most evident when blasting a nonmetallic coating or contaminate bonded to a metallic substrate .
• Dry Ice stripping should enable the removal of ceramic TC without modifying the MCrAlY bond coat characteristics and mainly the surface morphology.
• This method does not include the step of pre-demaging the ceramic coating before removing the ceramic layer using dry ice blasting as included instead in the previous cited patent [US20080178907 ] .
• the pre-damaging using shot peening or another sand blasting method using abrasive media shows the risk to damage the substrate characteristics as roughness and thickness.
• the only pre-heating is not able to pre-damage the ceramic coating.
• the pre-heating alone or the combination of pre-heating and quenching are not able to pre-damage or to remove the ceramic coating as a TBC or to get faster the dry ice stripping process.
• the only thermal shock is not able to remove the ceramic coating.
• Dry Ice Blasting is not able to remove in a fast way the ceramic coatings treated with shot peening or/and pre-heating as indicate in the previous cited patent [US20080178907 ] .
• Only a combination of a pre-heating during or immediately before the solid CO 2 blasting can lead to a fast ceramic coating stripping.
• This method includes only pre-heating by irradiation performed during or immediately before the blasting with solid CO 2 . This is due to the ceramic coating damage mechanism.
• the density of the solid CO 2 pellets is proportional to the shock wave power. The more the pellets are dense, the more the gas volume growing during the sublimation is greater, the more the shock wave is stronger.
• the equipment used in this invention is able to maintain high the density of the CO 2 pellets sudden out of the spray gun nozzle.
• the mass flow of the solid CO 2 pellets is proportional to the shock wave power. The more the amount of Dry Ice pellets interact with the coating surface subliming, the stronger is the shock wave.
• FIG. 1 shows a scheme of the ceramic coating to be removed (Fig.1(2)) by means of blasting by solid C O2 without damaging the substrate (Fig.l (1)) characteristics where the coating is deposited.
• Substrate characteristics are thickness and roughness.
• FIG. 2 shows in a plurality of sub-Figs. 2(a) and 2 (b) the two different how the pre-heating step is carried out during the solid CO2 blasting (Fig.2 (a)) or immediately before (Fig.2 (b) ) ; 3 schematizes the sand blasting gun during the stripping phase spraying the dry ice pellets schematized as 4. 5 schematizes the IR lamp used to obtain fast pre-heating of the substrate (1) /Coating (2) using IR radiation schematized as 6.
• FIG. 5 shows the Scheme of the CO2 Pellets Two-Hose Nozzle .
• the present invention refers to a method and an equipment to remove ceramic protective coatings 2 (i.e. Thermal Barrier Coating such a Yttria Partially Stabilized Zirconia - YPSZ) with high removal efficiency and without damaging the substrate 1 characteristics .
• ceramic protective coatings 2 i.e. Thermal Barrier Coating such a Yttria Partially Stabilized Zirconia - YPSZ
• the removing of the ceramic coating 2 without damaging the substrate 1 characteristics is obtained by a combination of coating/substrate pre-heating by irradiation immediately before (Fig.2 (b) ) or during the stripping (Fig.2 (a)) and improved solid CO 2 4 blasting parameters.
• the substrate 1 can be metallic, ceramic, plastic or composite.
• the substrate characteristics not affected by the present invention are the substrate thickness and roughness.
• Substrate thickness can vary in an range of 1 ym to 1 m.
• the substrate can be rough (Ra > 9 ym) or smooth (Ra ⁇ 9 ym) .
• the stripping method is a single stage process where only a combination of a pre-heating during or immediately before the blasting with solid CO 2 4 can lead to the ceramic coating stripping.
• the equipment to remove ceramic protective coatings is divided in two parts: Pre-heating Station 5 and Sand Blasting Machine Station 20.
• This method does not include the step of pre-damaging the ceramic coating 2 before stripping step by dry ice blasting.
• the substrates 1 coated with ceramic coating 2 are pre-heated in sequence in the pre-heating stations 5 (Fig.3) up to the maximum temperature that the substrate can tolerate.
• the pre-heating station 5 is able to heat the coating/substrate up to a maximum temperature of 1000°C.
• the coated component in moved in the solid CO 2 blasting station 20.
• the coating stripping using solid CO 2 blasting with optimized parameters is performed up to the quenching of the process at room temperature.
• the component is then moved in another pre-heating station 5 while another hot component is moved into the stripping station 20 (Fig.3) .
• the steps of pre-heating and solid CO 2 blasting are repeated for each substrate up to the complete coating removal.
• the Sand Blasting Machine Station 20 for blasting by solid CO 2 4 used in the method consists of a compressor, a feeder unit to feed dry ice into one or more spray guns 3.
• blast machines There are two general classes of blast machines as characterized by the method of transporting pellets to the nozzle: two-hose (suction design) and single-hose (pressure design) systems. In either system, proper selection of blast hose is important because of the low temperatures involved and the need to preserve particle integrity as the particles travel through the hose.
• two-hose system dry-ice particles are delivered and metered by various mechanical means to the inlet end of a hose and are drawn through the hose to the nozzle by means of vacuum produced by an ejector-type nozzle. Inside the nozzle, a stream of compressed air (supplied by the second hose) is sent through a primary nozzle and expands as a high velocity jet confined inside a mixing tube.
• this type of nozzle When flow areas are properly sized, this type of nozzle produces vacuum on the cavity around the primary jet and can therefore drag particles up through the ice hose and into the mixing tube where they are accelerated as the jet mixes with the entrained air/particle mixture.
• the exhaust Mach number from this type of nozzle is, in general, slightly supersonic. Advantages of this type of system are relative simplicity and lower material cost, along with an overall compact feeder system.
• Pellet blast machines are also differentiated into dry ice block shaver blasters and dry-ice pellet blasters.
• Pellet blast machines have a hopper that is filled with pre-manufactured CO 2 pellets.
• the hopper uses mechanical agitation to move the pellets to the bottom of the hopper and into the feeder system.
• the pellets are extruded through a die plate under great pressure.
• the pellets are available in several sizes ranging from 0.040 inch (1 mm) to 0.120 inch (3 mm) in diameter.
• the 0.120 inch (3 mm) in diameter pellets are commercially available.
• the solid CO 2 blasting machine station use a continuous (not pulsing) solid CO 2 blasting flow (constant pressure) .
• Said continuous flow is obtained using a core in the feeder device ( Figure 4) in combination with a spray two-hose nozzle gun 3 ( Figure 5) .
• the Dry Ice is fed using a special feeding device as schematized in Figure 4.
• the dry ice pellets contained in a box 9 are moved by a rotating scoop 10 in a large hole 11. Then the dry ice pellets are moved from the position 11 by a rotating punched tool in a further hole in the position 12.
• An air pressure flow (in a range of 1-5 bar) coming from 13 moves the pellets accumulated in 12 in the direction 14 up to the two-hose nozzle of the spray gun 3.
• the two-hose nozzle is schematized in Figure 5.
• Solid CO 2 pellets are fed in the main nozzle hose 17 by axial injection 16 in the internal injector 18.
• a second pipe with high pressure air up to thirty bar is connected with the convergent /divergent nozzle 19 (Fig.5) .
• the high pressure air is accelerated by convergent/divergent nozzle up to supersonic speed.
• the dry ice pellets 4 are injected directly in the accelerated high pressure air stream after the nozzle throat 19.
• the continuous flow shows a mass flow of solid CO 2 4 in a range of about 100 - 3500 g/min and a pressure in a range of 1 - 30 bar.
• a continuous flow of solid CO 2 is very important in order to reach very high removal rate. In fact if the flow is pulsed not only solid CO 2 will arrive on the coating surface but cool air, too. The cool air will decrease the substrate/coating temperature without a contribution to the stripping process that is due to the shock waves due to the solid CO2 sublimation. In that way the removal rate will be less than using a continuous flow of solid CO2.
• High pressure is very important to increase the mass flow and to increase the removal rate. In fact, when the shock waves crumble ceramic coatings, the high pressure aids ceramic fragments removal.
• the solid CO2 Pellets used for blasting have very high density (Density 1.4 - 1.6 g/cm 3 ) .
• the CO2 pellets density is very important because the more is the density, the more is the shock wave power due to the solid CO2 sublimation.
• the sand blasting equipment is designed to maintain the pellets density in a range of 1.525 - 1.6 g/cm 3 before the impact on the ceramic coating. This is obtained using in combination the above mentioned feeder and two-hose nozzle.
• the pre-heating systems is performed by IR lamps 6 using irradiation.
• the irradiation with IR allows to perform the preheating during the solid CO2 blasting;
• the irradiation with IR is able to heat the substrate/coating system up to 1000°C.
• the heating speed depend on the nature of the substrate and it can be in a range of l°C/min to 100°C/min.
• the pre-heating systems consist of IR lamps 6 in a range of wavelength of 1-10 ym and with a power output in a range of 1000 - 50000 W.
• the method as recited in claim 1 is able to remove a ceramic coating with a speed of 1 - 100 cm 2 /min.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Coating By Spraying Or Casting (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Processing Of Solid Wastes (AREA)

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• 2010
  • 2010-04-29 IT ITPR2010A000031A patent/IT1399945B1/it active
• 2011
  • 2011-04-27 CA CA2797184A patent/CA2797184A1/en active Pending
  • 2011-04-27 CN CN201180021417.0A patent/CN103108725B/zh active Active
  • 2011-04-27 WO PCT/IB2011/051839 patent/WO2011135526A1/en active Application Filing
  • 2011-04-27 EP EP11723670.3A patent/EP2576138B2/en active Active
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